A collection of animations constructed specifically for Honors 98 (Severe Weather). The movies are either animated GIFs for QuickTime movies. The QuickTime software can be found here. File sizes for the animations are listed; some of the animations are large.

Index to this page:

Simulations of convective rolls

Numerical simulation of boundary layer horizontal convective rolls (HCRs). Horizontal panel shows vertical motion at 250 m above ground level along with temperature departures (reds are warm) from the horizontal average. Vertical cross-section shown at right. Clicking on either image spawns 1.9 MB animated GIF in new window. Although the simulation starts with surface heating that varies in a spatially random way, the vertical wind shear causes coherent roll-type circulations to form. Source: after Fovell and Dailey (2001)

Horizontal plane (250 m above ground) Vertical plane

The sea and land breezes

As seen in a sophisticated mesoscale model: The summer season sea and land breezes in Southern California sometimes result in the formation of cyclonic flow (the Catalina eddy) over the bight, as seen in this picture of ground-surface temperature and near-surface winds forecasted for 10Z (3 AM) 9 June, 2002. Clicking on image spawns a 1.3 MB GIF animation spanning 48 hours and showing the formation of Catalina eddies on successive days. Source: R. Fovell, UCLA; made with the MM5 model

This much simpler model of the sea and land breeze still captures many of the circulation's salient features. This is a two dimensional model showing vertical cross-sections of temperature perturbation (reds are warm) and vertical velocity at left, and pressure perturbation (blues are low) and horizontal velocity at right. The time shown is 6 PM local; the domain is 8 km deep and several hundred kilometers wide. Clicking on the image spawns a 1.7 MB QuickTime movie spanning two days. Source: R. Fovell, UCLA, using Rotunno's (1983) heating function

Convective initiation

Initiation of convection in Oklahoma and Kansas on 12-13 June 2002. Visible satellite imagery. Click on image to spawn 2.3 MB GIF animation in new window. Source: NCAR JOSS


Initiation of convection by/at the sea-breeze front in Florida on 11 June 1997. Visible satellite imagery. Click on image to spawn 688 KB GIF animation in new window. Source: Unidata

Initiation of convection by/at the sea-breeze front in Florida on 2 July 1995. Visible satellite imagery. At the earliest time, rolls are ubiquitous over the land surface (see, especially, inside red circle) and convection has begun firing along the sea-breeze front (white arrow). Convection is well-developed in center and right images; note outflow boundary in latter (red arrow). Click on any image to spawn a 1.3 MB QuickTime movie. Source: NASA

1643Z 1843Z 2016Z

Simulation of convective initiation involving the sea-breeze and convective rolls: This is a vertical cross-section (60 km wide and 8 km deep) from a relatively simple three-dimensional model including both the sea-breeze and convective roll simulations. The sea-breeze front (SBF) marks the marine air boundary and a roll updraft located farther inland. As the SBF approaches the roll, deep convection is spawned. Clicking on the image spawns a 1 MB QuickTime animation. Look for the significant speed variations in the SBF; it both accelerates and slows during the period covered. Source: R. Fovell, UCLA, after Fovell and Dailey (2001)

Squall lines

Squall line extending from Kansas to Texas on 7-8 May 1995. Composite radar imagery. Note propagation to the east, development of extended area of light precipitation on west (back) side, and development of new convection ahead of the line. Click on image to spawn 2 MB GIF animation in new window. Source: A. Kankiewicz, Colorado State Univ.

2345Z #1

Numerical simulation of a multicellular squall line. Contours: vertical velocity; shaded: equivalent potential temperature. Note periodic development and rearward propagation of convective cells. Click on image to spawn 160 KB GIF animation in new window. Source: after Fovell and Tan (1998); made with the ARPS model

20220 sec

Numerical simulation of stratospheric gravity waves. The sequential development of convective cells results in a periodic disturbance of the overlying stable stratosphere, resulting in the generation of gravity waves (buoyancy oscillations). Vertical velocity contoured; temperature perturbations colored (warmed air in red). Click on image to spawn 748 KB GIF animation. Source: after Fovell, Durran and Holton (1992); made with the ARPS model

Effect of the subcloud cold pool on convection, part I. Image below shows a nonprecipitating storm in an environment with deep vertical wind shear. The storm leans "downshear", as expected. Such a storm would precipitate into its own inflow. Contoured: horizontal velocity; shaded: vertical velocity (upward motion in red). Click on image to spawn a 212 KB GIF animation in new window. Source: R. Fovell, UCLA

Effect of the subcloud cold pool on convection, part II. Subcloud cooling comes from evaporation of hydrometerors falling from the cloud. This cooling exerts a first-order effect on storm strength andf structure. Simulation at left is the control run, a mature multicellular storm with "upshear"-tilting circulation. The simulation at right shows how the control storm changes after deactivation of subcloud cooling. Clicking on either image spawns an 872 KB GIF animation. Note the storms start with precisely the same initial conditions. Source: R. Fovell, UCLA

Control simulation Subcloud cooling deactivated

Supercell storms and storm splitting

Rotating supercell storms are favored by environments having considerable vertical wind shear, and can form as a result of the splitting of preexisting convection. The simulation at left had only vertical wind speed shear. The original convective cell split into two new storms of equal strength but having opposite senses of rotation and very different trajectories. With directional shear (picture at right), one of the split storms is definitely favored. The reddish area is a rainwater isosurface and vectors indicate midtropospheric wind perturbations. Clicking on images spawn approximately 525 KB QuickTime movies. Source: R. Fovell, UCLA; made with the ARPS model

No directional vertical wind shear With directional vertical wind shear

Two views of a supercell. Both panels show surface temperature (colored) with near-surface winds. Left panel depicts 0.6 g/kg condensed water isosurface; right image presents isosurface of 20 m/s updraft velocity. Clicking on the image below spawns ~ 1.5 MB animated GIFs. Source: R. Fovell, UCLA; made with the ARPS model

Condensed water isosurface Updraft isosurface

Tornado? The image below zooms in on the lower portion of the rotating supercell storm shown above. The gust front is seen in both the surface temperature perturbation and surface wind field. The blue isosurface shows a envelope of rainwater; the orange isosurface encloses areas of large cyclonic vertical vorticity. Such vertically oriented vortex tubes develop sequentially along the gust front, growing out of low-level rotation, and build upwards towards the cloud. Clicking on the image below spawns a QuickTime movie; this movie is 5 MB. Source: R. Fovell, UCLA; made with the ARPS model


Hurricane Lili, 2-4 October 2002. Shown are enhanced IR and radar imagery and surface map for approximately 9Z (4 AM CDT) on October 3rd. Clicking on images spawn 2.8, 1 and 0.5 MB GIF animations in new windows. Sources: Enhanced IR and radar imagery from Unisys; surface maps from NCEP

Enhanced IR Radar Surface map

Two simulations of Hurricane Lili: At left, winds at 10 m along with estimated radar reflectivity at 14Z (9 AM CDT) for an MM5 simulation. Click on image to spawn a 664 Kb GIF animation. Right panel juxtaposes COAMPS and MM5 simulations, also at 14Z. Shown are winds at 4.5 km and cloud water field at 4 km. Clicking on image spawns 1.7 MB QuickTime movie. Note landfall is delayed for both simulated storms and storm track for MM5 simulation is less accurate. MM5 run also places largest precipitation in wrong quadrant.Source: R. Fovell, UCLA


Downslope winds and hydraulic jumps

Hydraulic jumps: Under certain conditions, air flowing over a mountain can be accelerated on the lee slope, resulting in very strong winds concentrated near the ground surface. The fluid there thins, as evidenced by the colored (potential temperature) field in the left-hand plot. Further downstream, however, the fluid can very suddenly become much thicker in an abrupt "hydraulic jump". The jump is very turbulent. The right-hand panel shows horizontal velocity perturbations zoomed in on the mountain. Clicking on the images spawns 396 and 224 KB GIF animations. Source: R. Fovell, UCLA; made with ARPS model

Potential temperature field Horizontal velocity perturbation field

Santa Ana winds in the Los Angeles basin are common in winter when high pressure builds in the cold high desert. The figure below shows surface temperature (colored) and 10 m wind vectors at 11Z (3 AM PST) for the January 6, 2003, high wind event. Clicking on the image spawns a 1.6 MB GIF animation. More on Santa Ana winds may be found at this link. Source: R. Fovell, UCLA; made with the MM5 model